1. Field of the Invention
The present invention relates to an audio processing apparatus and method, and in particular relates to an audio processing apparatus and method for microphone sensitivity calibration.
2. Description of the Related Art
There are numerous methods for a microphone array to process audio signals. For example, generalized sidelobe cancellation (GSC) is a popular method.
FIG. 1 shows a schematic diagram of a conventional audio processing apparatus using the GSC method. The audio processing apparatus 100 comprises a main microphone 110, a reference microphone 120, a fixed beamformer 130, an adaptive blocking filter 140 and an adaptive interference canceller 150. The main microphone 110 and the reference microphone 120 receive sounds from an audio source (not shown in FIG. 1) and, inevitably, noises from non-audio source sources, wherein the sounds are desired signals but the noises are not. The input signals generated by the main microphone 110 and the reference microphone 120 are further provided to the fixed beamformer 130 and to the adaptive blocking filter 140. The fixed beamformer 130 uses the GSC method, to extract the desired signals from the mixture of the sounds and the noises and generate a main channel output corresponding to the sounds, and the adaptive blocking filter 140 removes the desired signals from the mixture of the sounds and the noises and generates a reference channel output corresponding to the noises. Since there are always sidelobes in the main channel output due to leakages from the reference channel at different frequencies, the adaptive interference canceller 150 is coupled to the fixed beamformer 130 and the adaptive blocking filter 140 to compensate the main channel output and obtain the final output. After beamforming, the final output is provided to and processed by a Wiener post-filter to further reduce the stationary and non-stationary noises.
Performance of the GSC beamforming or for the following Wiener post-filtering depends on the perfect matching of the sensitivity of the two microphones 110 and reference microphone 120. The voice activity detectors (VADs) are implemented both in the adaptive blocking filter 140 and adaptive interference canceller 150 to avoid the cancellation the desired sound. Without reliable microphone sensitivity calibration, it is impossible for the VADs to provide correct information. However, sensitivity mismatch between microphones always occur. Moreover, since the GSC beamforming is implemented in the time domain and the sounds and the noises are mixed when they are received, it is hard for the GSC beamforming to remove all of the instantaneous interference. Thus, a new method to deal with the problematic issues described previously is needed.